Towards carbon sequestration using stainless steel slag via phase modification and co-extraction of calcium and magnesium

Iron– and steelmaking processes produce a large quantities of greenhouse gas and metallurgical slag. Using Ca/Mg–rich phases in the slag to capture and bind the CO₂ via mineral carbonation is a promising approach to reduction of emissions and solid wastes to be landfilled. However, Cr–bearing stainless steel slag (SSS) cannot straightforwardly be employed for carbon capture and storage (CCS) or rather carbon capture and utilization (CCU). For the dual–purpose of chromium immobilization and co–extraction of calcium and magnesium, a slag modification using added MnO is performed followed by an acid leaching treatment. Results show that the MnO content has a significant influence on the phase composition and element distribution of SSS. A Box–Behnken design (BBD) based acid leaching treatment of SSS is investigated and optimized. Second-order polynomial regression models that reveal a functional relationship between processing parameters and leaching yields of calcium and magnesium are established and verified by the analysis of variance (ANOVA). Model calculation results show a good agreement with the experimental data. The direct (linear) and cross-correlated effects of the processing parameters on the leaching yields are illustrated by three-dimensional (3D) response surfaces. The maximum leaching yields of calcium and magnesium obtained in this work are 65 % and 55 %, respectively, while for chromium the leached amounts are well below legislative limits.